1. Field of the Invention
The present invention relates to a tuning circuit having an amplitude-varying function and to an integrated circuit for a wireless communication device.
2. Description of Related Art
A common example of a tuning circuit and an AGC (Automatic Gain Control) in, for example, ASK (Amplitude Shift Keying) communication is shown in FIG. 9. The tuning circuit is configured by a LC parallel resonance circuit where an end of a coil (inductance) L1 and a capacitor (capacity) C1 are connected to a reference voltage Vref. The AGC circuit is configured by a variable gain amplifier, a rectifying circuit (REC) and a comparator (COMP). The variable gain amplifier adjusts and outputs the amplitude of an AC signal from the tuning circuit to an output terminal OUT. A gain amplifier, a detecting circuit and a waveform shaping circuit are connected to the output terminal OUT, and the amplitude-adjusted AC signal is processed (e.g., see Japanese Patent Application Laid-Open Publication No. Hei10-23084).
The gain factor of the AC (alternating current) signal in the variable gain amplifier is determined by the rectifying circuit and the comparator. That is, the amplitude of the AC signal is smoothed by the rectifying circuit to obtain a DC signal. Thereafter, the DC signal is compared with a reference voltage VAGC by the comparator. As a result of this comparison, the comparator feeds back output for lowering the gain factor to the gain amplifier in a case where, for example, the amplitude of the AC signal is excessive.
The tuning circuit and the AGC circuit are used, for example, in receiving devices of remote control systems. Remote control systems have various applications, such as opening/closing and locking the doors of vehicles and houses, and also starting and stopping engines in vehicles.
In realizing AGC functions such as, for example, conducting automatic control to lower the amplitude of an AGC signal in a case where the amplitude of the AGC signal is large, power consumption is great when an analog control system including the variable gain amplifier and the rectifying circuit is used. When a circuit that consumes a large amount of power is used in a battery-driven receiving device of a remote control system, battery drain is accelerated.